Malaysia Goodson, of Stamford, Connecticut, fell down the stairs at a midtown Manhattan station at around 8 p.m. Monday, police said.
Goodson was unconscious when police arrived. She was taken to a hospital, where she was pronounced dead. Her child was unharmed.
Goodson's brother, 23-year-old Dieshe Goodson, told Eyewitness News that his little sister was like a twin and that she "died trying to protect her baby."
The baby of their four siblings, Malaysia "always watched out for people," he said.
"She was very protective," he said. "I'm going to be honest, I don't know what to do. She was my best friend."
It was not immediately clear whether the 22-year-old Goodson had a medical issue that precipitated her death or whether she died from impact. The city's medical examiner will determine the cause of death.
The baby is now in the custody of her father and grandfather.
The MTA issued the following statement:
"This is an absolutely heartbreaking incident. While the ultimate cause of the event is being investigated by the MTA, medical examiner, and the NYPD, we know how important it is to improve accessibility in our system. The Fast Forward Plan acknowledges and prioritizes this work as one of four key priorities, and aims to ensure that riders will never be more than two stops away from a station with an elevator. This will be accomplished through the addition of up to 50 elevators over the next five years. We believe this is an important issue of practicality and equality, and once accomplished, riders will never be more than two stops away from a station with an elevator."
The 7th Avenue/53rd Street station, which services the B, D and E lines, does not have an elevator, according to the New York Times.
Only about a quarter of New York City's 472 subway stations have elevators, and the elevators that do exist are often out of service. Parents and caregivers who ride the subways with young children become adept at carrying a stroller and child up and down the stairs. Bystanders will often pitch in to help, but not always.
Shams Tarek, an MTA spokesman, said the Seventh Avenue B-D-E station where Goodson fell does have escalators. Parents are generally advised not to ride escalators with strollers, however.
The lack of elevators makes it difficult for people in wheelchairs to use New York's subway system, and disability-rights activists have staged frequent protests over the issue.
"The lack of accessibility in our subways is literally killing people," New York City Council Speaker Corey Johnson tweeted.
The lack of accessibility in our subways is literally killing people. I am heartbroken by this tragedy, and am keeping this family in my thoughts. NYC must do more for families and the disabled. https://t.co/X2w91JdKtD— NYC Council Speaker Corey Johnson (@NYCSpeakerCoJo) January 29, 2019
Andy Byford, who was appointed the MTA's head of the subway system last year after running the Toronto Transit Commission, has set a goal of adding enough elevators to the system that after five years, no rider will be more than two stops away from an accessible station.
WABC-TV contributed to this report